1. Field of the Invention
The present invention relates to a structure for preventing electric field concentration to prevent the influence of an electric field from a conductive layer such as wiring provided on the surface of a semiconductor device from causing the electric field concentration on the surface of a semiconductor layer formed under the conductive layer.
2. Description of the Prior Art
FIGS. 13A and 13B are plan and cross-sectional views showing a structure for preventing electric field concentration in a conventional semiconductor device, respectively. The cross-sectional view of FIG. 13B is taken along the line A--A of the plan view of FIG. 13A.
As shown in FIG. 13B, on the surface of an n.sup.- semiconductor substrate 1 are formed a p diffusion region 2 and an n.sup.+ diffusion region 3 at a distance. A conductive layer 4 such as wiring is in contact with the n.sup.+ diffusion region 3 and is insulated from the n.sup.- semiconductor substrate 1 and the p diffusion region 2 by an insulating layer 5. In the insulating layer 5 formed immediately under the conductive layer 4, conductive plates 6a-6e are disposed in alignment. The leftmost conductive plate 6e is contacted by the p diffusion region 2, and the other conductive plates 6a-6d are held floatingly in the insulating layer 5. The edges of the adjacent conductive plates overlap each other.
The conductive plates 6a and 6b, 6b and 6c, 6c and 6d, and 6d and 6e are capacitively coupled respectively. The conductive layer 4 and the respective conductive plates 6a-6e are also capacitively coupled.
When the potential at the p diffusion region 2 is low and the potential at the n.sup.+ diffusion region 3 is high, a depletion layer extends from a pn junction interface of the n.sup.- semiconductor substrate 1 and the p diffusion region 2. By the depletion of the almost entire n.sup.- semiconductor substrate between the p diffusion region 2 and the n.sup.+ diffusion region 3, high breakdown voltage is achieved. Dotted lines in FIG. 14 indicate equipotential lines in the depleted region of the n.sup.- semiconductor substrate 1 in the case where the conductive layer 4 is absent. Since the depletion layer can extend laterally without restriction on the surface of the n.sup.- semiconductor substrate 1, electric field concentration does not occur on the surface of the n.sup.- semiconductor substrate 1.
On the other hand, when the conductive layer 4 having a high potential is present, the lateral extension of the depletion layer on the surface of the n.sup.- semiconductor substrate 1 is restrained by the influence of an electric field from the conductive layer 4 if there are not the conductive plates 6a-6e. The equipotential lines in the depleted region of the n.sup.- semiconductor substrate 1 are indicated by dotted lines in FIG. 15. The electric field concentration occurs on the surface of the n.sup.- semiconductor substrate 1 and the breakdown voltage is deteriorated. In order to prevent the above occurrence, the conductive plates 6a-6e are provided.
Provided that coupling capacitances between the conductive plates 6a-6e and between the conductive layer 4 and the conductive plate 6a are sufficiently larger than coupling capacitances between the conductive layer 4 and the conductive plates 6b-6e, the respective potentials at the conductive plates 6a-6e ascend in order from the conductive plate 6e toward the conductive plate 6a according to the capacitance division of potential differences between the p diffusion region 2 and the n.sup.+ diffusion region 3. Thus, the depletion layer extends easily rightward (on the n.sup.+ diffusion region 3 side) on the surface of the n.sup.- semiconductor substrate 1, and the electric field concentration on the surface of the n.sup.- semiconductor substrate 1 is relieved.
According to the structure for preventing electric field concentration in the conventional semiconductor device as above constructed, the coupling capacitances between the conductive plates 6a-6e and between the conductive layer 4 and the conductive plate 6a must be sufficiently larger than the coupling capacitances between the conductive layer 4 and the conductive plates 6b-6e to relieve the electric field concentration on the surface of the n.sup.- semiconductor substrate 1 effectively. For the achievement, however, the insulating layer 5 which intervenes between the conductive layer 4 and the conductive plates 6b-6e must be much thicker than insulating layers generally used, and the formation of the insulating layer 5 is difficult. For this reason, the effect of preventing electric field concentration on the surface of the n.sup.- semiconductor substrate 1 cannot be sufficiently achieved when the insulating layer 5 with an easily formable thickness is used.